1. Field of the Invention
The present invention relates to a display device used in a portable information device such as a cellular phone or a mobile computer.
2. Description of the Related Art
Conventionally, a liquid crystal display device that is small and thin and consumes less power is often used as a color image display device used in a portable information device such as a cellular phone or a mobile computer.
The liquid crystal display device includes a backlight as an illumination device. One known type of backlight is an edge-lit illumination device in which light from a light source enters on a side surface of a light guide plate and output from a top surface (hereinafter, referred to as exit surface) of the light guide plate. The light source includes a point light source such as a cathode ray tube or a light emitting diode (LED). The light guide plate has a surface (hereinafter, referred to as opposing surface) opposite to the exit surface, and a number of grooves and dot patterns are formed on the opposing surface. Further, the exit surface often includes a diffusing pattern formed thereon, which has an effect of diffusing light. An incident surface (that is, a surface which faces the light source and on which the light from the light source enters) of the light guide plate includes a prism formed thereon. The backlight with the above-mentioned structure has a function of outputting surface light because the light from the point light source is diffused by the prism. A material for the light guide plate may include a transparent resin such as polycarbonate (PC) or acryl (PMMA), which has a higher refractive index than air. In addition, a structure is predominantly adopted, in which a diffusing sheet or a prism sheet is provided on the exit surface side of the light guide plate. Further, a reflecting sheet is provided below the light guide plate.
There has also been developed a light guide plate that is specific to a point light source such as an LED and uses a microprism array so as to function without a prism sheet (see, for example, JP 2007-280952 A (FIG. 6)).
The conventional backlight structure uses the prism sheet and hence has limits in thickness, luminance efficiency, and cost. In the case of the microprism array backlight proposed in JP 2007-280952 A, the pyramidal diffusing patterns are basically oriented in the same direction, and hence the light from the point light source is not fully exploited. With the conventional backlight structure, the luminance efficiency is at substantially the same level as the case where the prism sheet is employed. Moreover, utilization efficiency of the light from the light source of the backlight is very low.
In addition, the conventional microprism array backlight is only adapted to a single point light source, and it has been impossible to increase the number of the light sources to two or more.
Further, the structure without the prism sheet has a problem in that moiré (interference) is likely to occur when a liquid crystal panel is placed thereon.